The purpose of this project is to determine the natural history of the recently discovered and isolated causative agent of Lyme disease and related disorders. The relationship(s) between the spirochete and its various tick vectors (Ixodes dammini, I. pacificus, I. ricinus, Amblyomma americanum) is being determined by establishing through conventional as well as transmission and scanning electron microscopy (a) the development of the spirochete within the ticks, and (b) the mode(s) of transmission to vertebrate hosts. In cooperation with outside agencies, tick/spirochete surveys are being conducted to determine prevalence of infected ticks in endemic foci. Similarly, the natural source(s) for infecting ticks is being evaluated first serologically (indirect immunofluorescence) and subsequently through recovery of spirochetes from serologically implicated hosts. The western deer tick, Ixodes pacificus, has been confirmed as the vector of Borrelia burgdorferi in western U.S. Of 1,647 adult ticks from California and Oregon, 25 (1.4%) contained spirochetes indistinguishable from the Lyme disease agent. The lone star tick, Amblyomma americanum and the black-legged deer tick Ixodes scapularis, have been shown experimentally to maintain and transmit B. burgdorferi; these ticks must be considered potential vectors in southern and southeastern U.S. The cotton rat (Sigmodon hispidus) appears to be susceptible to the Lyme disease spirochete and experiences spirochetemias typical for relapsing fever spirochetes. For patients with neurological abnormalities (aseptic meningitis, encephalitis, neuritis, radiculitis, etc.) Lyme disease should be considered in a differential diagnosis. A hitherto undescribed borrelialike spirochete was detected in the argasid tick, Ornithodoros coriaceus and is considered a potential cause of epizootic bovine abortion (EBA).